1. Field of the Invention
This invention relates to a beam splitter disposed rearwardly of the picture-taking lens for dividing the light from the picture-taking lens.
2. Description of the Prior Art
A beam splitter having a dielectric material multilayer film deposited by evaporation on a transparent substrate such as a glass substrate is usually used as a quick return beam splitter for a single lens reflex camera.
This beam splitter has an advantage that since it has substantially the same reflection factor in any portion thereof, an object image relatively excellent in resolution as compared with that in a beam splitter of the type hereinafter described is formed on the focusing plate of the viewfinder optical system. However, this beam splitter has a polarizing characteristic and this has led to the possibility that a metering error arises when the light from the object is a polarized light.
Beam splitters are also known in which non-evaporated portions in the form of spots or slits are provided on the surface of a transparent substrate and a metal film is deposited by evaporation on the portions of the surface other than the non-evaporated portions (Asahi Camera, July, 1963, P. 215, Asahi Camera, July, 1965, pl. 232, Japanese Patent Publication No. 29793/1969, U.S. Pat. No. 4,081,807 and Japanese Laid-open Patent Application No. 119030/1978). In such beam splitters, the portion on which a metal film is formed acts as a reflecting surface and the portion on which the metal film is not formed acts as a light-transmitting surface.
Such beam splitters have no polarizing characteristic unlike the previously mentioned beam splitter and therefore do not give rise to the problem of metering errors resulting from polarized light. However, such beam splitters have light-transmitting portions in the form of spots or numerous slits and this leads to the disadvantage that the resolving power is reduced. Further, in such beam splitters, if an attempt is made to increase the quantity of transmitted light, various disadvantages would arise. That is, if the number of the light-transmitting portions is made constant and the size thereof is increased to increase the quantity of transmitted light, there would occur a disadvantage that the pattern in the form of spots or numerous slits can be observed through the viewfinder optical system and, if the size of the light-transmitting portions is made constant and the number thereof is increased, there would arise a disadvantage that flare is created by diffraction or scattering and the resolving power of the object image is reduced.